(a) Field of the invention:
The present invention relates to an optical system for projecting information such as graduations on a diaphragm dial or shutter speed dial within a field of view of a finder of single-lens reflex photographic cameras comprising a pentagonal prism in said finder.
(B) Description of the prior art:
There have conventionally been known a variety of methods for observing an image of information such as graduations on diaphragm dials and shutter speed dials within a field of view of finders of single-lens reflex cameras, said methods being roughly classified into two types. One type of method is to project graduations on a diaphragm ring or shutter speed ring within a field of view of finder by using an appropriate optical system. Such an optical system functions, as illustrated in FIG. 1A, in such a manner that optical information of graduations on a shutter speed ring or diaphragm ring indicated by chain lines is reflected on surfaces 2a and 2b of a prism 2 so as to follow an optical path a, whereafter said optical information is passed through a pentagonal prism 1 for observation within the field of view of a finder. Speaking more concretely, said optical information is made visible at the position indicated by symbol D which is located above the field of view F as shown in FIG. 1B. The other type of method is to project an image of graduations on special dial interlocked with a diaphragm operating means or shutter speed operating means within the field of view of a finder. Of these two methods, the former does not require a mechanical interlocking system and is hence more advantageous than the latter. However, the former method produces difference in dioptor between the filed of view in the finder and information image which can hardly be eliminated. FIG. 2A and FIG. 2B illustrate an example of optical system wherein said defect has been eliminated by inserting several miniature lenses in the optical path for matching the dioptor between the field of view in the finder and information image. However, such an optical system has drawbacks that it is rather difficult to manufacture in practice, that image information is minimized and that it is impossible to optionally select image position in the finder. The second method has defects that it requires a disc or film graduated with diaphragm aperture divisions or shutter speed divisions as well as a mechanical system for interlocking said graduation disc with the diaphragm ring, etc., thereby unavoidably making mechanisms of the photographic cameras very complicated.